1. Field of the Invention
The present invention relates to an image forming apparatus applied to a printing unit of a digital copying machine, a facsimile or the like as well as to a digital printer, a plotter or the like, for forming an image on a recording medium by jetting a developer.
2. Description of the Background Art
An image forming apparatus outputting image signals as a visible image on an recording medium such as a sheet of paper has been known, as disclosed, for example, in Japanese Patent Laying-Open No. 6-91918. In the image forming apparatus, an electric field is exerted on charged particles so that the particles jet out or travel by the electric force, and by changing potentials applied to a control electrode including a plurality of passage holes arranged along the jet travel path, the charged particles are attracted and adhered on the recording medium, whereby the image is directly formed on the recording medium.
Another image forming apparatus has also been proposed in which a control electrode is provided on an insulating substrate surface near an opposing electrode when a distance between a toner carrier and the insulating substrate is short, and a control electrode is provided on an insulating substrate surface closer to the toner carrier when the distance is long, so that the distance between the control electrode and the toner carrier is made as constant as possible.
In the conventional image forming apparatus having the above described structure, means for controlling passage of charged particles through a gate is used. In such an image forming apparatus, whether charged particles (developer) are allowed to jet out or not is controlled by controlling an electric field generated between the gate and a carrier carrying the developer, and by a strong electric field generated by an opposing electrode, the developer is attracted and reaches the surface of the sheet, which is the recording medium.
In the above described image forming apparatuses, an amount of charged developer which travels or jets is controlled by the electric field formed between the gate and the carrier.
Therefore, when the electric field has different strength, the amount of charged developer which jets differs.
In such an image forming apparatus, when a cylindrical sleeve is used as a charged toner carrier and a control electrode having two-dimensional gate arrangement is used, the distance between the sleeve and the control electrode cannot be made constant because of the curvature of the cylinder, and at an end of the sleeve, the distance to the control electrode is larger than at the central portion.
Accordingly, at the end portion, the generated electric field is weaker, and therefore, the amount of developer passing through the gate and the course of travel are not constant. Therefore, the density of the dots formed at the end portion is low, resulting in unsatisfactory contrast, while the dot density is high at the central portion.
In view of the foregoing, such attempts have been made, as increasing a potential applied to the control electrode at an end portion when the developer is passed. However, in that case, the number of power supplies necessary for adjusting the potential for controlling jetting of the developer is increased. In addition, if difference in the potentials exceeds breakdown voltage of an FET (Field Effect Transistor) used for a circuit for switching potentials, an FET having higher breakdown voltage must be provided separately. This unavoidably leads to increased cost of FETs as well as of necessary measure for insulating the circuit from the high voltage. Therefore, the number of parts and the size of the apparatus are unavoidably increased.
An approach to control jetting of the developer without increasing the breakdown voltage of the FET experiences the following problem.
In order not to increase the breakdown voltage of the FET, height of a potential applied to the control electrode must be limited. Therefore, when a high potential is applied as a potential preventing jetting (hereinafter referred to as OFF potential), then a potential necessary for jetting (hereinafter referred to as ON potential) must be set sufficiently low, which prevents satisfactory jetting of the developer, resulting in blurred image with low contrast.
Conversely, when a high potential is applied as the ON potential, the OFF potential must be set sufficiently low and, in that case, prevention of jetting of the developer will be insufficient, causing a fog. Accordingly, satisfactory contrast cannot be obtained, resulting in poor image formation.
Further, in a color image forming apparatus, not only dot formation but also color reproductivity are degraded as desired toner jetting cannot be ensured. As a result, image quality degrades.
In view of the foregoing, an attempt has been made to change the timing of applying potential to the control electrode in such an image forming apparatus as described above. When this approach is taken, it becomes unnecessary to change the potential to the control electrode in the image forming apparatus. On the other hand, it becomes necessary that a mechanism for changing the timing of potential application is provided for every control electrode. This unavoidably leads to increased cost and size of the apparatus, as the number of parts is increased.
In the image forming apparatus described above, it is desirable that an electric field formed between the carrier and the opposing electrode is kept constant. For this purpose, a method has been known in which the insulating substrate is configured to conform to the curvature of the carrier sleeve, so that the distance between the carrier and the opposing electrode is made constant and the electric field therebetween is kept constant.
However, it is difficult to arrange the carrier and the insulating substrate with high precision. If the arrangement is not sufficiently precise, it is possible that the insulating substrate comes into contact with the surface of the toner carrier, destroying a developer layer carried thereon. In that case, in addition to the problems described above, the state of the developer layer could be changed, and therefore desired control of developer jetting is impossible, preventing satisfactory image formation.
Further, in this case also, color reproduction would be insufficient in a color image forming apparatus, resulting in image degradation. Therefore, such arrangement of the insulating substrate in a practical apparatus has been difficult.
In the above described image forming apparatus, in order to make constant the electric field formed between the carrier and the opposing electrode at an end portion and at the central portion, it may be possible to adjust the positional relation between the carrier and the control electrode such that the relation is the same at the end portion and the central portion. For this purpose, the control electrode is arranged at the insulating substrate further from the carrier of the developer passage gate at the central portion, while at the end portion, the control electrode is arranged on the insulating substrate nearer to the carrier of the developer passage gate. This is another attempt to solve the problem of uneven electric field and resulting difference in toner density.
In that case, it is necessary to provide separate control systems for controlling the control electrodes for the central portion and the end portion on the insulating substrate or it is necessary to provide interconnections for the control electrodes on both surfaces of the insulating substrate. This makes the apparatus complicated and increases the number of parts and cost. In addition, if it becomes necessary to provide ICs for controlling the control electrodes on both surfaces as interconnections are provided on both surfaces of the insulating substrate, attachment of the apparatus to an image forming mechanism would be difficult as compared with attachment simply on one surface.